The present invention belongs to the field of the production of structures.
More particularly, the invention relates to the production of thin and stiffened panels made of composite material.
In the field of structures, and more particularly of aeronautical structures, the use of structures that are strong and as light as possible is appreciated.
In particular, the structures such as airplane fuselages, or the shell structures in general, are constructed, among other things, from panels in which a skin that is relatively thin compared to the other dimensions of the panel is stabilized by stiffeners (also called stringers) fixed to the skin which increases the strength of the skin and above all which increases its rigidity and gives it an acceptable resistance to buckling.
Such stiffened panels can be made of metallic materials or, as is becoming more generally commonplace, in particular in the aeronautical field, of composite materials.
The type of composite materials used for these applications generally consists of a stack of plies comprising fibers, often long fibers, held in a hard organic matrix.
The fibers are oriented in directions that can be different according to the ply concerned in order to confer on each part made of composite material mechanical properties suited to the directions of the forces introduced into the part.
The parts made of composite material are shaped, to obtain the geometry that the parts need to have in the planned application, in a step during which the organic matrix, generally an organic polymer resin, is not hard, either the resin is a resin that hardens in a subsequent baking process, so-called thermosetting resins, or the resin is a resin in a plastic state when the temperature is raised to a sufficient value, so-called thermoplastic resins.
To produce a stiffened panel made of composite material, in particular of thermosetting composite material, it is necessary for the stiffeners to be strongly attached to the skin of the panel, and, for this, there are a number of techniques which are implemented these days.
A first technique consists in independently producing the skin and each of the stiffeners made of composite material and in joining the stiffeners to the skin in a hard state. The joining is then done by means that are also used with metallic materials such as bonding or riveting.
A drawback with this first technique lies in the fact that it is lengthy to implement and that each of the elements to be joined has to be produced with sufficient accuracy to ensure that the joining will be correct. In practice, to achieve the necessary accuracy, said accuracy demands the use of molds which are both costly and specific to each part to be produced.
A second known technique consists in producing the stiffeners made of composite material in a first step and then, in a second step, producing the skin made of composite material directly on the prepositioned stiffeners, and finally, in a third step, proceeding with the hardening of the skin at the same time as said skin adheres to the stiffeners.
This technique makes it possible to guarantee that the skin will be applied very accurately to the stiffeners, but it requires the stiffeners to be perfectly held in position in a mold during the production of the skin and it is difficult in this case to obtain the accuracy and the surface finish expected of the finished panels.
In a variant of this technique, the skin is produced first and the stiffeners are produced on the skin in a second step, but, while in this case it is generally easier to obtain the surface finish sought on the face of the panel opposite that to which the stiffeners are fixed, it is more difficult on the other hand to obtain a satisfactory accuracy in the positioning of the stiffeners.
This second technique and its variant further require the interposition of a film of glue at the skin/stiffeners interface and therefore an additional step in the production of the panels.
A third technique consists in simultaneously molding the skin and the stiffeners in a non-hardened state and in carrying out the simultaneous hardening of the skin and of the stiffeners during a baking step during which the stiffeners are joined to the skin in a phase of polymerization of the resin.
This technique, called cocuring, allows for a very efficient joining of the stiffeners and of the skin but is demanding in terms of molds which have to be of dimensions similar to the panel to be produced and it has proven difficult to control the dimensions of the part when the tolerances to be observed are tight.